A beautiful crypt designed by Francesco Borromini himself is preserved beneath the church of San Carlino.

Like most church crypts, the hypogeum of San Carlino had a structural purpose, as a foundation for the church and to protect it from damp.

But it also had a functional purpose: as a cemetery. For many centuries, the ecclesiastical and social custom of burying the dead in or around churches existed, until the end of the 18th century, when, for health reasons, it became necessary to bury in cemeteries located outside the cities and in open fields. The crypt of San Carlino was used as a cemetery until the Napoleonic period.

From a structural point of view, the crypt of San Carlino is striking in that it exactly matches the floor plan of the upper church and therefore, lacking the trompe l'oeil of the church's design, it clearly shows how small the church is.

Also, like the upper church, the side walls of the crypt have figuratively arranged 'four fau00e7ades' facing inwards: four door-like openings flanked by niches that give the impression of windows.

Also striking is the entrance to the crypt, in which the rectangular, static opening is 'sectioned' by the circular, dynamic structure of the cantilevered spiral staircase leading to it.

The crypt is roofed by a dome that is very difficult to build, as it is a very low structure with lunettes and rests on non-rectilinear walls. Only Borromini's skill was able to overcome the difficulty of such a construction with the materials of the time.

The crypt contains a room that Borromini designed as his tomb. But given his tragic death, the result of a long agony after a failed suicide attempt, he decided not to use the crypt of San Carlino, his great architectural work, and preferred to be buried next to Carlo Maderno, his distant relative, great friend and distinguished master.

It is a small room, roofed by a small dome and illuminated by a skylight that allows light to enter from the side fau00e7ade of the convent.